Research On The Carrier Recovery Techniques In WiMAX

This paper includes two main parts. One part does research on QAM carrier recovery techniques in digital microwave links; While the other part talks about OFDM carrier recovery techniques in WiMAX.In the 3G base station interconnection project cooperated with Powercom company, we based our plan on QAM modulation for frequency efficiency. High order QAM modem designs are highly effected by fading in the channel, signal distortion and other problems because of the characteristics of QAM signals. The phase error and phase jitter of the recovered carrier are very sensitive to the SNR failure of the QAM system. So it is a key problem to do the carrier recovery precisely. We first arise three main problems in QAM carrier recovery. The first problem is that the recovery loop must have a acquisition range enough to chath the biggest frequency error; the second is to reach a phase jitter small enough to meet high level QAM modulation requirements; the third is to emiliate the nπ/2 phase ambiguity. To solve the three problem, the paper utilize a structure of a FD and a PD and emiliate the nπ/2 phase ambiguity using the differential coding. The FD uses RCFD algorithm. RCFD has similar performance as GFD, while it costs less in realization, because it realize the time match filter and frequency match filter in one structure. The PD uses improved DD structure, this algorithm adds an error estimator in the loop to control the gain factor later so as to control the noise. in the loop. The simulation shows that the carrier recovery loop has large acquisition range up to 300KHz, small static phase jitter up to 5KHz, and have improved performance in low SNR performance.OFDM is a multicarrier modulation techinique, it has high frequency efficiency, anti multipath distortion and many other andvantages. One of the key problems in OFDM system design is that OFDM system has high requirements about carrier synchronization. Carrier offset has two main influence on OFDM systems: on one hand it makes the constellation rotates; on the other hand, it destroys the orthogonality between carriers, so causes inter carrier distortion. The second part of the paper first introduces several classic OFDM carrier recovery algorithms and then introduces our OFDM carrier recovery system in three small parts: fractional error estimation, integral error estimation and phase error estimation. The fractional error estimation part analyzes the performance of SCA and conjugate symmetry algorithm. After comparing the performance of the two algorithms, we choose SCA, which has a larger estimation range and smaller error. The integral estimation part utilizes only one preamble symbol and takes the advantage of the relativity of the PN code to estimate the error. The phase error estimation part utilize one pilot tone to track the phase error and uses the estimation after compensation structure to improve precision.